Originally Posted by xlcrlee

It would be good to forestall a stess riser by somehow tapering the carbon strip instead of ending it abruptly. Or?

I am not worried about that because the carbon strip ends at the polyhedral joint which is a major stress riser anyway, especially with a 1/16" plywood braces on each side of the spars. Incidently, I found that the carbon strip is about 4 grams per meter, and the 1/8" square spar is around 5.5g grams per meter for each one.

You are right: I should have looked closer, but now I see the polyhedral joint! I am in the habit of repeatedly bringing this subject up with Chinese ME's, so that was a reflex response on my part.

That is good looking work, John. I can see that two pounds for my sailplane if built like that would be a low figure. I would consider a 1/8" sq bottom spar at the rear since I am a tension freak. The pictures are great, please do more. Charles

Originally Posted by xlcrlee

You are right: I should have looked closer, but now I see the polyhedral joint! I am in the habit of repeatedly bringing this subject up with Chinese ME's, so that was a reflex response on my part.

Actually the photo is misleading because I only glued one of the two carbon pieces in when the photo was taken. What you can see in the pic is the center dihedral joint. In the picture, the second carbon strip is sitting on the plan just in front of the wing waiting to be glued in next.

In my previous work I often dealt with component specs for Chinese manufacturers. Actually a mixture of electronic and mechanical specs. It is so easy for things to get misinterpreted, especially with the language barrier, so we can't be too careful.

The rear spar is 1/8" balsa just to support the covering at the 65% chord point. This is part of the airfoil design and you can see the same support on the Alegro-lite 2m wing. Maybe the wing is a bit over-engineered, but because it is quite thin I wanted to be on the cautious side. I am also thinking of making a light-weight bungee for launching.

Here are pictures of the Quaker wing which is 54" and weighs less than 4 ounces. It is tough for a docile flyer.

Charles, The quaker wing is pretty but personally I don't think the airfoil is likely to be efficient enough for a glider. I think a glider airfoil needs to have a smooth profile from the leading edge over the top of the spar. It looks as though the covering is going to be stretched between the leading edge and the spar, so it seems there will be a discontinuity in the curve profile at the leading edge and also at the spar. I am guessing that adding leading edge sheeting will give a lot more maximum lift and better efficiency as well.

The gentle lady wing has a curve discontinuity at the leading edge, but has a spar placment that does not come in contact with the covering. That is another alternative if you don't mind the old style single spar layout. Personally I think that LE sheeting is the way to go. For my latest wing I have the sheeting all the way back to 45% of the chord, as recommended by the designer of the AG series airfoils.

Yes, John, I like the 45% sheeting and plan on using it. I showed the wing because of the airfoil, the four spar arrangement, the gusseted TE and the wash out. The light weight with a little more strength than the average is my goal in a canard floater. I want it to float like the Playboy in the slightest lift. There is always the fear of losing a model like that, but it it will show the benefits of building light. The Ultra Stick is almost finished and the the canard glider will be next.. Charles

Here's an update on my progress so far. The wing is nearly done except the dihedral joint and covering. Maybe everone knows this already, but this is the first time I have made ribs by transfering the image from a photocopy using an hot iron. The rib designs were generated from Profili software.

The two halves as pictured weigh 70g together, which isn't particularly light. The rib spacing on the tip panel has been increased to 50mm, whereas on the center panel 45mm spacing is used. I did this to increase the span of the tip panel as a late design change. Its not an optical illusion!

There won't be any more progress for a while because I will be away for the next two weeks for Chistmas and some travelling.

I'm up early John. Thanks for showing that pretty piece of work. Did the rib copy come from the ink jet printer? How did you get multiple copies on a page? Looks like it saved a lot of work. Best wishes for your Christmas break. Charles

My printer is a monochrome laser copier/printer. The toner in the laser printer or photocopier is heat activated, which allows this to work. I don't expect an inkjet printer will work, so I think you would need to get photocopies of them.

Profili allows many airfoils to be printed in a single sheet, but to make the spacing even closer I cut them and ironed them individually for each rib. It takes some pressure to make the transfer work so is used the edge of the iron slowly across each parts of the printing. It still takes some time and effort but less so than any other method I know. The accuracy is generally a lot better as well.

I hope you have a happy and safe christmas break also.

WOW John! That is some clean work!

Hi Don and many thanks for your excellent information.

I am designing a large electric powered sailplane and have a question about spars, but please, rule of thumb or approximations is as close to the truth as I need to get!

So here are the relevant details: Span 18 ft, weight 10 lbs, stressed to 5 G at the central wing joiner (using 5/16 aircraft aluminum vertical bars bolted with AN Hardware at both ends of the overlap in the fuselage, and twice in each wing root)

Centre sections (both sides) use ice-hockey stick handles as spars. These are shaved to the height of the wing section, but left at full thickness, and could extend to the tip joiner point at 4 ft out from the fus. I have personally bounced up and down on these handles (my weight =220 lbs) when they are suspended between two concrete blocks, and they show no signs of failing. In the vertical sense the sticks as spars have a solid hard-wood core (which splits longitudinally quite easily) and, laminated to each side is seven ply reinforcing 1/4" thick.
Obviously this is WAY too much spar at the 4 ft point, and beyond. So I intend to reduce the thickness as quickly as possible by simply sawing the spar down in thickness while maintaining as much depth as possible (blending the ends of the cuts to avoid stress risers).
After the 4ft point I intend to use spruce top and bottom caps and balsa sheer webs, again tapered in thickness and height to match the tapering wing section.
So the question is, how far from the fus can I reduce the spar thickness by say, 10%, 25%, 50%, 75% (or other amounts as you see fit)? Or not if you think I need all of it all the way to the 4 ft point.

I assume that the total 5G load will be shared equally by each wing (in a pull up maneuver) so each wing has to provide 25 lbs of support, but how is this load distributed from centre to tip? Is it a linear progression, subject to an approx inverse sqaure rule or some other general rule of thumb?

Lastly thanks so much for your time, and I wish you and all my readers a very Happy Holidays!

John

Originally Posted by ckuklbac

...
I am designing a large electric powered sailplane and have a question about spars, but please, rule of thumb or approximations is as close to the truth as I need to get!

I assume that the total 5G load will be shared equally by each wing (in a pull up maneuver) so each wing has to provide 25 lbs of support, but how is this load distributed from centre to tip? Is it a linear progression, subject to an approx inverse sqaure rule or some other general rule of thumb?
...
John

Hi John,

I found this in one of my German books about RC-sailplane design.

It shows the rectangular wing, the (nearly linear) lift distribution and the resulting bending moment from lift, which the author used to calculate the max loads. The max. load case was a pull up maneouvre at max. speed. The plane used as an example was a saiplane with 4 m (~12 ft) span and weight of 3,2 kg (~ 7 pds).

Interestingly he got load factors of up to n=32 !

He further asumes, that half of the lift A/2 is "concentrated" on each wing at app. 1/4 of the wingspan b. The weight of the wing itsself is distributed along the wing, so no bending moment from it. Just the weight of the fuselage G(Rumpf) * load factor n.
(sorry, a little bit more then a rule of thumb, just for translation purposes)

One of his conclusions is, that usually nobody designs a plane for that max. load case (just too heavy). Another example in his book was an aerobatic sailplane with 2,4 m span. There he came to a max. allowed load factor of 9.6.

The book is very interesting, as the author (Franz Perseke) not only calculates, but also made experiments with different materials (balsa, plywood, spruce...) to get material data. Unfortunately I only have a partly copy of that book.

Merry Christmas to all
Uli

As I see this is the canard corner and anything related will fit,
I'd link another design, not new, but hasn't been mentioned here so far, I think.

http://www.f5d.org/modelle/felix2.gif

It's from the very early days of electric pylon racing.

biber